Generally, a diesel particulate filter (DPF) is used for reducing particulate matter from a diesel engine. In conventional diesel particulate filters, PM is deposited on surfaces of cell walls, which causes a pressure loss of an exhaust gas to be increased rapidly, since the PM is trapped on the cell walls. Therefore, a system, in which an overheat combustion treatment at 600° C. or more by fuel injection is periodically operated in order to make DPF self-recovering, has been suggested (refer to Japanese Patent Unexamined Publication No. H06-294315 and Japanese Patent Unexamined Publication No. H08-189339).